What happened
On September 28, 1992, two SA 315B "Lama" helicopters operated by Helog were conducting concrete transport operations between Goldau and a construction site on Mount Rigi. The accident involved helicopter HB-XCN, piloted by Pilot Y. After several flights had been suspended due to poor visibility, operations resumed in the early afternoon.
At approximately 14:25, with improved weather conditions, the pilot of HB-XCN began a flight carrying a concrete container suspended by a 16.8-meter sling assembly. Shortly after takeoff, at around 14:30, witnesses observed the helicopter flying at a very low altitude. The concrete container struck the top of a tree or obstacle, causing the heavy load to swing violently backward.
The momentum of the slung cargo pulled the steel cable taut against the rotating tail rotor. The cable severed all three tail rotor blades almost simultaneously. Immediately following this mechanical failure, the helicopter entered an uncontrolled forward flight attitude, pitched up sharply, and rolled backward three times before crashing into a gorge below. The aircraft was destroyed, and the pilot was killed instantly.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the wreckage, witness statements, and forensic evidence. Key findings included:
- The tail rotor blades were severed symmetrically near their roots, consistent with being cut by a high-tension steel cable in flight.
- The cargo sling assembly was found 50 meters from the main wreckage, indicating it had detached or been pulled away during the crash sequence.
- Forensic analysis confirmed that the damage to the tail rotor and sling components occurred during the accident sequence, not during recovery.
- The pilot held a valid commercial helicopter license with extensive experience on the SA 315B type. Medical examination revealed no evidence of alcohol, drugs, or pre-existing medical conditions that could have contributed to the accident.
- Weather conditions at the time were generally favorable, with visibility exceeding 10 km, though local terrain and wind gusts were factors.
Findings
The investigation determined that the primary cause of the accident was the loss of control due to the severing of the tail rotor blades by the cargo sling. This occurred because:
- The helicopter was flying at a low altitude over uneven terrain.
- The concrete container struck an obstacle (likely a tree), causing the load to swing backward with significant force.
- The long, rigid steel section of the sling acted like a whip, striking and cutting the tail rotor blades.
- The loss of tail rotor authority resulted in immediate loss of yaw control and subsequent aerodynamic instability that could not be recovered from.
Contributing factors included the low flight height which reduced margin for error during low-level operations and the aerodynamic properties of the sling assembly, which increased drag and reduced the angle between the fuselage and the load, making it more susceptible to swinging backward upon impact.
Safety action
The report highlights the critical importance of maintaining safe clearance heights when operating with external loads in mountainous terrain. It underscores the need for pilots to be aware of how cargo dynamics can change rapidly during low-level flights, particularly when obstacles are present. No specific new regulatory recommendations were issued in this summary, but the findings serve as a case study in the dangers of low-altitude sling load operations.